Modern internal combustion engines have a controlled catalytic converter for which the oxygen concentration in the exhaust of the internal combustion engine is measured by a lambda probe and used to control the internal combustion engine in order to minimize pollutant emissions of the internal combustion engine.
The lambda probes used for this purpose include binary (narrowband) types. One aspect of lambda probes of this kind is that they contain a ceramic whose surface is able to determine the oxygen concentration in the exhaust gas of the internal combustion engine, a corresponding voltage signal being fed out on an output line. Another aspect is that such lambda probes incorporate a heater to which a voltage is applied via heating wires in order to heat the lambda probe up to operating temperature, as lambda probes of this kind only operate properly in a particular temperature range.
During operation of the above-described heated lambda probes, various fault scenarios can arise which will be described briefly below.
In one fault scenario, the heater incorporated in the lambda probe is too weak to heat up the surface of the ceramic in the lambda probe to the required operating temperature or rather to maintain said operating temperature. The lambda probe then cools down and is therefore no longer functionally capable of correctly measuring the oxygen concentration in the exhaust gas of the internal combustion engine.
In another fault scenario, the output lines of the lambda probe are open-circuited, e.g. because of a wire break in the supply lead to the surface of the ceramic in the lambda probe, resulting in complete functional failure of the lambda probe.
Finally, in a third fault scenario the heating of the lambda probe by the integral lambda probe heater is insufficient, for operational reasons, to heat up the ceramic in the lambda probe to its operating temperature or maintain it at said operating temperature. This operation-related fault scenario can arise, for example, in the event of a cold start or when there is a momentary risk of water hammer, when the heating output is reduced for component protection. This scenario can also occur if the heater characteristic map is filled with incorrect data.
However, the legal exhaust gas regulations and in particular the exhaust gas regulations of the CARB (California Air Resources Board) require that the exhaust gas purification system must commence controlled operation as quickly as possible in the case of both a cold start and a warm start of the internal combustion engine. Otherwise, a time is specified within which a fault must be detected and stored in order to document that the exhaust gas regulations have not been complied with.
It is therefore known from the prior art to test the operability of the lambda probe so that when the internal combustion engine is cold-started there is a maximally fast transition to controlled operation of the exhaust gas purification system as soon as the lambda probe has been heated up to operating temperature. For this purpose, the output voltage and the internal resistance of the lambda probe can be measured. The hardware configuration of the lambda probe is such that, in the event of a wire break, the lambda probe voltage is held at a fixed potential and the Nernst cell internal resistance of the lambda probe diverges.
The problem with this conventional testing of the operability of the lambda probe is the fact that it is not possible to differentiate between the different fault scenarios described above. For example, weak lambda probe heating is manifested by a similar output voltage and internal resistance behavior of the lambda probe to that resulting from the above described wire breakage.